The fabrication and implementation of electronic circuits involves electrically connecting devices through specific electronic paths. In particular, very short intra-transistor circuit electrical paths are commonly formed. For example, a drain or source of a transistor can be electrically connected to the gate of the same transistor. Also, other very short path length electrical circuits are commonly formed. Short path length electrical connections are commonly referred to in the art as local interconnects.
Present processes for fabricating such local interconnects presents some problems which have not yet been successfully addressed in the industry. The following few diagrams illustrate some of the more significant problems identified in state of the art local interconnect manufacturing processes.
FIG. 1A is a simplified schematic depiction of a single transistor formed on a silicon wafer in accordance with the present state of the art. The following depicted process is a conventional process used to form a local interconnects structure. The depicted structure includes a wafer having a transistor 100 formed thereon. The transistor is generally isolated from other transistors and circuit structures on the wafer using isolation structures. In the depicted structure, the isolation is accommodated using shallow trench isolation (STI) structures 110. Also, the transistor and associated substrate are generally covered with one or more etch stop layers 111 which facilitate fabrication. Over the entire substrate is an inter-layer dielectric material 112, also known in the industry as an ILD.
Continuing with the description of FIG. 1A, the transistor 100 includes a gate portion 101 having a top gate contact 101c formed thereon. The gate contact 101c is configured to facilitate electrical contacts to the gate 101. Also, the gate 101 commonly has spacers 102 arranged on the sides to enhance electrical performance. Additionally, the transistor 100 includes a drain region 103d and a source region 103s, each of which include an associated electrical contact (104s and 104d respectively). To establish a local interconnect the gate 100 can, for example, be electrically connected with the source 103s. Such connection is typically achieved by electrically connecting the gate contact 101c with (in this case) the source contact 104s. FIGS. 1B, 1C, and 1D, 1E, 1F illustrate aspects of one conventional process for achieving this connection and some of the drawbacks of existing methodologies for its construction.
In order to establish such a local interconnect, portions of the ILD 112 and the etch stop 111 must be selectively removed. In one commonly employed conventional process (depicted by FIGS. 1B and 1C) a portion of the substrate is masked and then the surface is etched. For example, a photo resist layer 120 is patterned to include an opening 121 that will define the location of the local interconnect. A standard etch process (i.e., a plasma etch process) is used to remove the ILD 112. When the etch process reaches the etch stop layer 111 located over the gate 101, the etch process slows significantly over the gate 101. However, the etch process continues relatively unimpeded in the region over the source 103s. Unfortunately, the etch protection provided by the etch stop layer 111 is not perfect, and the etch stop layer 111 is eroded while the process removes the ILD 112. Thus, the etch stop layer 111 becomes thinner on top of the gate 101 and also in the regions near the top of the exposed spacer 102. These thickness variations lead to some significant process difficulties which were not as important when larger feature sizes were used (e.g., 1 micron technologies). However, for deep sub-micron feature sizes (e.g., for 0.18 micron CMOS technologies or smaller) certain process difficulties have a more pronounced effect on the resultant semiconductor structures.
FIG. 1C schematically depicts a simplified cross section view of a final etch profile resulting from a conventional process. The ILD 112 is removed in the opening 121. In a subsequent etch using an alternative chemistry, the etch stop layer 111 is also removed in the opening 121. However, as previously hinted at, the removal of the ILD 112 in one portion of the etched surface before the remainder of the ILD is removed means that while the remaining portions of the ILD layer are being etched away, the same etch process is working on the exposed etch stop layer 111. This leads to uneven erosion of the etch stop layer 111. Consequently, when the unevenly eroded etch stop layer 111 is etched away the substrate under the prematurely thinned etch stop layer 111 can be come damaged. This happens because the etch process is designed to remove all of the etch stop 111, including the thickest (un-eroded) layer of etch stop 111. However, where the etch stop has been previously eroded the etch process begins to etch the underlying layers. This is a very undesirable outcome. Additionally, circuit design and process technologies extend manufacturing technologies into the deep sub-micron range the processes become extremely sensitive to over etch problems. This is particularly problematic for feature sizes of less than 0.15 micron and especially so as process technologies extend below the sub 0.09 micron (90 nm) range.
FIGS. 1D, 1E, 1F illustrate some issues presented by conventional manufacturing processes used today. One difficulty is illustrated in the simplified depiction of FIG. 1D. As explained above, when the ILD is removed from the gate 101 some ILD remains above the source 103s. Therefore, when ILD etching continues to remove the ILD above the source 103s some of the etch stop layer 111 is removed in the uncovered regions. Thus, as shown, the etch stop layer 111 is thinner above the gate 101 and also above the spacer 102. In a conventional process, etch selectivity between the ILD etchant and the material comprising the etch stop layer 111 reduces this damage. However, due to ever shrinking critical dimensions even the relatively small damage done to the etch stop 111 during this ILD etch is significant enough to pose a serious problem. When the etch stop 111 is etched away, all of the thin portion 111g of the etch stop 111 will be gone, whereas some portion of the thicker layer 111s will remain. Subsequent etching will remove the rest of the thicker layer 111s, but, some portion of the substrate underlying the thin portion 111g of the etch stop 111 (here, for example, the source 103s and portions of the exposed spacer 102) will also be removed. This can have drastic effects of the performance of the resulting circuit. For example, the gate electrode 107 can be etched resulting in serious fluctuations in the gate current.
Additionally, the spacer 102 can be eroded 102e as shown in the simplified FIG. 1E. This is especially problematic near the top of the spacer 102. Such erosion can result in altered transistor parametrics.
FIG. 1F is a simplified depiction of a condition that can occur using conventional processes where an overetch condition caused by ILD thinning and excessive etching occurs. For example, the described overetch can etch through the etch stop 111 into the STI 110 creating an STI overetch condition that deepens the STI recess 113. This can cause increased junction leakage that, among other problems, can lead to higher power consumption. As can be expected, these and other related conditions, are highly undesirable and moreover, increasingly difficult to address using conventional processes.
For the reasons stated above, as well as other reasons, there is a need in the art for alternative process methods capable of establishing local interconnects used in integrated circuits.